This invention relates generally to an on-board system and method for detecting fuel vapor leakage from an evaporative emission space of an automotive vehicle fuel system.
A known on-board evaporative emission control system for an automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of the fuel tank by the volatilization of liquid fuel in the tank and a purge valve for periodically purging fuel vapors to an intake manifold of the engine. A known type of purge valve, sometimes called a canister purge solenoid (or CPS) valve, comprises a solenoid actuator that is under the control of a microprocessor-based engine management system, sometimes referred to by various names, such as an engine management computer or an engine electronic control unit.
During conditions conducive to purging, evaporative emission space that is cooperatively defined primarily by the tank headspace and the canister is purged to the engine intake manifold through the canister purge valve. A CPS-type valve is opened by a signal from the engine management computer in an amount that allows intake manifold vacuum to draw fuel vapors that are present in the tank headspace and/or stored in the canister for entrainment with combustible mixture passing into the engine""s combustion chamber space at a rate consistent with engine operation so as to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
Certain governmental regulations require that certain automotive vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline, have evaporative emission control systems equipped with an on-board diagnostic capability for determining if a leak is present in the evaporative emission space. It has heretofore been proposed to make such a determination by temporarily creating a pressure condition in the evaporative emission space which is substantially different from the ambient atmospheric pressure, and then watching for a change in that substantially different pressure which is indicative of a leak.
It is believed fair to say that there are two basic types of diagnostic systems and methods for determining integrity of an evaporative emission space against leakage.
Commonly owned U.S. Pat. No. 5,146,902 xe2x80x9cPositive Pressure Canister Purge System Integrity Confirmationxe2x80x9d discloses one type: namely, a system and method for making a leakage determination by pressurizing the evaporative emission space to a certain positive pressure therein (the word xe2x80x9cpositivexe2x80x9d meaning relative to ambient atmospheric pressure) and then watching for a drop in positive pressure indicative of a leak.
Commonly owned U.S. Pat. No. 5,383,437 discloses the use of a reciprocating pump to create test pressure in the evaporative emission space.
A reed switch is disposed to sense reciprocation of the pump mechanism, and serves both to cause the pump mechanism to reciprocate at the end of a compression stroke and as an indication of how fast air is being pumped into the evaporative emission space. The frequency of switch operation provides a measurement of leakage that can be used to distinguish between integrity and non-integrity of the evaporative emission space.
Commonly owned U.S. Pat. No. 5,474,050 embodies advantages of the pump of U.S. Pat. No. 5,383,437 while providing certain improvements in the organization and arrangement of that general type of pump. More specifically, the pump of U.S. Pat. No. 5,474,050: enables integrity confirmation to be made while the engine is running; enables integrity confirmation to be made over a wide range of fuel tank fills between full and empty so that the procedure is for the most part independent of tank size and fill level; provides a procedure that is largely independent of the particular type of volatile fuel being used; provides the pump with novel internal valving for selectively communicating the air pumping chamber space, a first port leading to the evaporative emission space, and a second port leading to atmosphere; and provides a reliable, cost-effective means for compliance with on-board diagnostic requirements for assuring leakage integrity of an evaporative emission control system.
The other of the two general types of systems for making a leakage determination does so by creating in the evaporative emission space a certain negative pressure (the word xe2x80x9cnegativexe2x80x9d meaning relative to ambient atmospheric pressure so as to denote vacuum) and then watching for a loss of vacuum indicative of a leak. A known procedure employed by this latter type of system in connection with a diagnostic test comprises utilizing engine manifold vacuum to create vacuum in the evaporative emission space. Because that space may, at certain non-test times, be vented through the canister to allow vapors to be efficiently purged when the CPS valve is opened for purging fuel vapors from the tank headspace and canister, it is known to communicate the canister vent port to atmosphere through a vent valve that is open when vapors are being purged to the engine, but that closes preparatory to a diagnostic test so that a desired test vacuum can be drawn in the evaporative emission space for the test. Once a desired vacuum has been drawn, the purge valve is closed, and leakage appears as a loss of vacuum during the length of the test time after the purge valve has been operated closed.
In order for an engine management computer to ascertain when a desired vacuum has been drawn so that it can command the purge valve to close, and for loss of vacuum to thereafter be detected, it is known to employ an electric sensor, or transducer, that measures negative pressure, i.e. vacuum, in the evaporative emission space by supplying a measurement signal to the engine management computer. It is known to mount such a sensor on the vehicle""s fuel tank where it will be exposed to the tank headspace. For example, commonly owned U.S. Pat. No. 5,267,470 discloses a pressure sensor mounting in conjunction with a fuel tank roll-over valve.
One generic aspect of the present invention relates to a method of initializing an evaporative emission space of a fuel storage system of an automotive vehicle preparatory to performing a leak detection test on the evaporative emission space, the vehicle being powered by a fuel-consuming engine and including an evaporative emission control system for purging fuel vapor from the evaporative emission space to the engine for combustion therein during conditions conducive to purging, the method comprising: creating a differential between pressure in the evaporative emission space and atmospheric pressure sufficient to perform a leak detection test; varying, over time, the created pressure differential within a range of differential pressures sufficient to perform a leak detection test; and then isolating the evaporative emission space from communication with both the engine and atmosphere, and performing a leak detection test.
Another generic aspect of the present invention relates to an engine-powered automotive vehicle evaporative emission control having a fuel storage system comprising an evaporative emission space for containing volatile fuel vapors generated by the volatilization of fuel in the storage system and a purge valve for purging fuel vapors from the evaporative emission space to an engine for combustion therein during conditions conducive to purging, including a leak detection system for detecting leakage from the evaporative emission space which comprises: a selectively operable prime mover for pumping gaseous fluid with respect to the evaporative emission space; a selectively operable valve which operates to a first condition for allowing the prime mover to move gaseous fluid with respect to the evaporative emission space, and to a second condition disallowing the prime mover from moving gaseous fluid with respect to the evaporative emission space; and a sensor providing an electric signal related to pressure in the evaporative emission space for controlling operation of the prime mover and of the valve; the leak detection system initializing the evaporative emission space preparatory to performing a leak detection test by causing the pump to create a differential between pressure in the evaporative emission space and atmospheric pressure sufficient to perform a leak detection test, including varying, over time, the created pressure differential within a range of differential pressures sufficient to perform a leak detection test, and then isolating the evaporative emission space from communication with both the engine and atmosphere and performing a leak detection test.
The foregoing, and other features, along with various advantages and benefits of the invention, will be seen in the ensuing description and claims which are accompanied by drawings. The drawings, which are incorporated herein and constitute part of this specification, disclose a preferred embodiment of the invention according to the best mode contemplated at this time for carrying out the invention.